1. Technical Field of the Invention
The present invention relates to the field of electronic circuits comprising transistors configured in differential pairs. Such configurations, comprising bipolar or MOS transistors, are often used in radiofrequency applications owing to their low sensitivity to interference, to their speed and to their limited interference effect on the power supply and the substrate in the case of integrated circuits.
2. Description of Related Art
It will be recalled here that an npn-type bipolar transistor having an n-doped collector, a p-doped base and an n-doped emitter, operates in the following manner. When the transistor is in unsaturated conducting mode, in other words currents can flow and the potential at the collector is higher than at the base, the potential at the base being itself higher than at the emitter, then the current Ie in the emitter is equal to the sum of the current Ic in the collector and the current Ib in the base, and: Ic=Is (exp(Vbe/Vt)−1) with Is the saturation current and Vt a voltage of about 25 mV. To a first order, the transistor behaves like a switch that is controlled by its base voltage. Thus, Ic=βIb, where β is the current gain of the transistor, of the order of 100.
Therefore, a flaw in this switch is the existence of the current Ib in the control signal which is absorbed by the base. Another flaw is that the potential on the emitter depends on the base potential and on the emitter current Ie. The variation of this potential follows a logarithmic curve. The difference in potential between a low and a high current will be about 200 mV.
Now, if the potential at the collector is lower than at the base, the transistor is in saturation mode. In this condition, the above relations are no longer valid. The transistor then accumulates electrical charges in the manner of a capacitor. When it returns to the unsaturated conducting mode, a non-negligible time is required to remove these charges which causes a delay in the switching.
The transistor is considered to be in the off-state when the voltage Vbe is zero. When returning to conducting mode, it requires a certain time to turn it on, which introduces a delay.
A need accordingly exists to provide a solution to the shortcomings explained above.